A display using a thin film light-emitting element of a self-light-emitting type, which emits light by itself when current is applied, has been extensively developed.
Such a thin film light-emitting element emits light by connecting an electrode to a single-layer or multilayer thin film formed using one or both of an organic compound and an inorganic compound when current is applied. Such a thin film light-emitting element is expected to reduce the power consumption, occupy smaller space, and increase the visibility, and the market is also expected to expand further.
It has become possible to manufacture an element which emits light more efficiently than before by dividing the function for each layer of a light-emitting element having a multilayer structure (for example, see Reference 1: Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987) by C. W. Tang et al.).
A thin film light-emitting element having a multilayer structure has a light-emitting stack provided between an anode and a cathode. The light-emitting stack includes a hole-injecting layer, a hole-transporting layer, a light-emitting layer, an electron-transporting layer, an electron-injecting layer, and the like. Among these layers, all the hole-injecting layer, the hole-transporting layer, the electron-transporting layer, and the electron-injecting layer may not necessarily be used depending on the element structure.
The hole-injecting layer in the light-emitting stack as above is formed with a material which can inject holes relatively easily from a metal electrode into a layer mainly containing an organic compound. The electron-transporting layer in the light-emitting stack is formed with a material that is superior in electron-transporting properties. Thus, each layer in the light-emitting stack is formed by selecting a material superior in each function.
However, a material mainly containing an organic compound, to which electrons can be injected relatively easily from an electrode, or a material mainly containing an organic compound which can transport electrons at a predetermined mobility or more is very limited. As is clear from the limitation on the material, the injection of the electrons from the electrode into the layer mainly containing the organic compound is originally rare to occur. Thus, the drive voltage is high. Further, an experiment shows that an element of higher drive voltage has higher drive voltage over time.